(a) Field of the Invention
The present invention generally relates to a fuel cell system for vehicles and a method for controlling the same. More particularly, the present invention relates to a fuel cell system for vehicles and a method for controlling the same which stably maintains an output of a fuel cell by precisely estimating a recirculated hydrogen amount to a fuel cell stack.
(b) Description of the Related Art
A fuel cell system is a well known type of electric generator system which converts chemical energy of a fuel directly into electrical energy.
The fuel cell system includes a fuel cell stack for generating electrical energy, a fuel supply for supplying a fuel (that is, hydrogen) to the fuel cell stack, an air supply for supplying oxygen in air (which is an oxidizing agent for the electrochemical reaction) to the fuel cell stack, and a device for managing heat and water for radiating reaction heat of the fuel cell stack to an exterior of the system and controlling operating temperature of the fuel cell stack.
Therefore, the fuel cell system generates electricity by an electrochemical reaction of the hydrogen, which is the fuel, and the oxygen in the air, and exhausts heat and water which are by-product of the reaction.
The fuel cell stack as applied to a fuel cell vehicle includes a plurality of unit batteries arranged sequentially. Each unit battery includes a membrane-electrode assembly (MEA) disposed at the innermost part thereof, and the membrane-electrode assembly includes an electrolyte membrane for transferring hydrogen ions, and catalytic layer. In particular, a cathode and an anode are spread at both sides of the electrolyte membrane so as to react the hydrogen with the oxygen. In addition, a gas diffusion layer (GDL) is positioned at an exterior portion of the membrane-electrode assembly (MEA). In particular, a GDL Is positioned at the exterior portion in which the cathode and the anode are positioned. Further, a separator is positioned at an exterior of the gas diffusion layer. The separator is formed of a flow field for supplying the fuel and the air to the cathode and the anode and for exhausting water generated by the reaction.
Thus, in a fuel cell, the hydrogen and the oxygen are ionized by a chemical reaction at each catalytic layer such that the hydrogen undergoes an oxidation reaction so as to generate hydrogen ions and electrons, and oxygen ions undergo a reduction reaction with the hydrogen ions so as to generate water. In particular, since the hydrogen is supplied to the anode (which is also referred to as an “oxidation electrode”) and the oxygen (or air) is supplied to the cathode (which is also referred to as a “reduction electrode”), the hydrogen supplied to the anode is ionized into hydrogen ions (H+) and electrons (e−) by the catalyst of the electrode layer formed at both sides of the electrolyte membrane. After that, only the hydrogen ions selectively pass through the electrolyte membrane, which is a cation-exchange membrane, and is transferred to the cathode. Simultaneously, the electrons (e−) are transferred to the cathode through the gas diffusion layer and the separator, which are conductors.
Therefore, the hydrogen ions supplied to the cathode through the electrolyte membrane and the electrons supplied to the cathode by the separator react with the oxygen in the air supplied to the cathode by an air supply so as to generate water.
At this time, movement of the hydrogen ions causes electrons to flow through an exterior conducting wire thereby generating current. When the water is generated by the reaction, heat is also generated.
In order to apply such a fuel cell system to the vehicle, it is important to maintain an output of the stack stably, and for this purpose the amount of recirculated hydrogen provided to the stack should be precisely detected or estimated.
However, since a gas recirculated in a fuel cell vehicle has a high water content, and since the gas passes through a short recirculation passage, it is difficult to directly detect the recirculated hydrogen amount by means of a flowmeter.
Therefore, the recirculated hydrogen amount has been typically estimated by using a thermal equilibrium equation. According to the thermal equilibrium equation, heat acquired from the supplied hydrogen is the same as heat lost by the recirculated gas. According to this thermal equilibrium equation, if the detected values (e.g., temperature, pressure, hydrogen concentration, and so on) are precise, a reliable recirculated hydrogen amount may be calculated.
However, the prediction of recirculated hydrogen amount using the thermal equilibrium equation depends greatly on temperatures. In particular, the prediction of the recirculated hydrogen amount using the thermal equilibrium equation is precise when equilibrium temperature is reached. However, the time required to reach equilibrium temperature is very long in an actual system. Therefore, a method for predicting the recirculated hydrogen amount by using the thermal equilibrium equation cannot be applied to an actual system.
In addition, since various heat generations/losses, such as heat generated by a blower, expansion heat of the hydrogen, condensation heat of mixed gas, and heat loss in the line, exist in an actual system, taking these effects into consideration makes the calculation very complex.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.